This invention relates to a method of producing a polyolefin microporous membrane and a separator for a lithium ion battery.
Polyolefin microporous membranes are being used as a separator for a lithium ion battery, a precision filter membrane, a separator for a capacitor, and a material for a fuel cell. Use as a separator for a lithium ion cell used for a small-size electronic device such as portable telephones and notebook type personal computers as well as an automobile battery is attracting particular attention. This invention relates to a method of producing such polyolefin microporous membranes and separators for a lithium ion battery comprising polyolefin microporous membranes produced by this method.
As examples of method for producing such polyolefin microporous membranes as described above, Japanese Patent Publication Tokkai 10-50287 disclosed a molding method by using a mixture of polyolefin resin with inorganic powders such as titanium oxide, aluminum oxide and potassium titanate and Patent Publication WO2006-038532 disclosed a molding method by using a mixture of polyolefin resin with inorganic particles of oxides and nitrides of silicon, aluminum and titanium.
Since the inorganic powders and particles that are mixed with polyolefin resin in these prior art production methods for polyolefin microporous membranes hardly have any elasticity for enabling them to follow the drawing process carried out in the molding step for the microporous membrane and since their compatibility with polyolefin is not good, the molding process for the microporous membranes was very cumbersome and the microporous membranes obtained as a result developed various troubles such as pin holes and voids. In particular, the mechanical strength of the resultant microporous membranes is not sufficient and their thermal stability at high temperatures is inferior.